Apple seeds contain amygdalin, a substance that releases cyanide into the blood stream when chewed and digested.The amygdalin is accessible only if the seeds have been crushed or chewed; a whole unbroken seed will pass right through.
Britannica also says:
…the average adult would need to eat anywhere from 150 to several thousand crushed seeds (depending on the apple variety) to be at risk of cyanide poisoning.
This answer isn’t clear, so how many apples (not seeds), assuming the eater chews the seeds would they have to eat to die from a lethal dose?
There are usually 5 seed pockets per apple.